ST. PETERSBURG, Fla. — Joe Girardi said the Yankees won’t know how Masahiro Tanaka’s right elbow is progressing until the injured pitcher gets into a game.

Yet before that happens, Tanaka has to graduate from throwing on flat ground, which he did Friday, to throwing from a mound. That will happen Saturday.

“Everything has been positive so far, he says he feels good, but you really don’t know,’’ the manager said before Friday night’s 5-0 loss that dropped the Yankees to eight lengths back of the first-place Orioles and 4 1/2 back of the second wild card.

Tanaka, who long-tossed in the outfield and went through fielding drills and pick-off moves from the mound Thursday, will throw 20-to-25 pitches off a mound Saturday. It will be the first time on the hill for Tanaka since July 8, when he suffered a small tear in the ulnar collateral ligament in his right arm. He is attempting to rehab it instead of having Tommy John surgery, an operation that would shelve the $175 million investment for 12-to-18 months.

Of course, Tanaka isn’t going from a bullpen session to a rehab game. If mound sessions progress without a problem, the next step is batting practice. After that it could be a simulated game or a minor league rehab game.
Still, Saturday is a big test.

“If I can’t throw the way I want to on the mound or in the bullpen, there is no way I will be able to throw in a game,’’ Tanaka said through a translator. “So, yes, definitely, the bullpen would be important.’’

The severe nature of the injury, the time of the year — minor league regular seasons are finished at the end of this month — and the risk/reward factor all play into the decision of whether or not Tanaka will return this year, has Tommy John surgery or continues to rehab instead of pitching this season.

Yet Girardi said the Yankees are going through the rehab process thinking they are getting Tanaka back at some point this year.

“I sure hope so, that’s why we are going through it,’’ Girardi said. “You have to find out if it’s the proper thing to do and, is his arm going to hold up? Because you’d hate to shut him down the whole year and go through it next year, too.’’
Girardi isn’t concerned that Tanaka won’t be truthful with the medical staff if a problem surfaces.

“He is smart enough to understand that, he told us when it happened,’’ Girardi said. “They have had the conversations and I think he will be able to tell them. There aren’t too many guys who can hide that.’’

As the Yankees try to get back in the AL East race or cop the second wild-card spot, they do it without a legitimate ace, which Tanaka quickly developed into.

Friday night’s starter, Brandon McCarthy, has been the best starter recently for a rotation that is made up of a returning Michael Pineda, Hiroki Kuroda, Shane Greene and Chris Capuano because CC Sabathia, Ivan Nova and Tanaka are on the DL.

They have held their own, but none is anywhere close to the pitcher Tanaka was when he went on the DL with a 12-4 record and a 2.51 ERA in 18 games.

Saturday the Yankees and Tanaka take the first substantial step in finding out if he will pitch again, require surgery or more rehab.